Seals prevent or mitigate leakage of one fluid into another. They may be selectively applied i.e. a flow may be allowed until a certain condition is reached and it is desirable for such fluid flow to be halted.
In a wireline well intervention environment, one application of such seals and sealing arrangements is wireline valves. Wireline valves or “WLVs” are backup valves used on wellbores when wireline is present in the event of wireline based intervention in the wellbore.
Wellbores frequently have highly flammable fluids at elevated temperatures and pressures within them. In the event of a mechanical problem on the wireline, such as stranded wire, the WLVs may be closed around the line to seal the wellbore allowing remedial work to be performed on the section of line above the valve.
Such wireline valves may have to seal a well for a prolonged period of time, typically more than 12 hours, until such times as the wireline and associated equipment can be repaired or replaced, the wellbore is permanently sealed or some other intervention made.
There are two general types of wireline valve rams: the line sized and shear/seal ram types.
Line sized rams, be they multi- or specific line size configuration, enable the wireline valve to close around static cable when activated. The rams contain rubber elements which, when actuated by the hydraulic actuators, apply such rubber pressure around the cable that an effective seal is created. Combined with a second set of rams, and standard viscous grease injection into the intermediate void, an effective well barrier is created through the application of rubber pressure around the cable and grease filling the intermediate voids inside the cable.
Shear/Seal type rams combine a cutting element at the front of the rams with blind seals to create an effective well barrier after the line has been cut and dropped.
Ram wireline valves typically have two gate-like members which, in a normal operating position, are positioned either side of a central bore of a wireline valve and upon actuation are forced together to prevent fluid flow.
The seals of a wireline valve may have to withstand the elevated temperatures and pressures of the wellbore fluid.
Rubber or some other elastomeric materials are often optimal sealing materials, as their resilience and imperviousness may produce good sealing arrangements. However, they may not be best suited to the high temperatures and pressures, and the seals may yield and flow and eventually fail if subjected to such conditions for extended periods.
The seals of either type will often be reinforced by attaching an elastomeric material to a metal reinforcing plate, either on one side, or more typically, by sandwiching the elastomeric material between two metal plates.
Typically, the wireline valve will have a wireline, rod or pipe suspended through it, and this wireline, rod or pipe may be attached to downhole tools or monitoring equipment. The wireline valve seals are usually adapted such that upon activation, a tight seal is formed around these wirelines, rods or pipes but they do not sever them nor is a good seal prevented by their presence.
In a wireline valve ram, the mating seal faces of the metal plates will usually include complementary channels positioned parallel to the wireline (for example) such that when the blowout preventer is activated, the channels form bores in the adjoined metal plates that the wireline is encased within. The elastomeric seal bodies will initially have a planar mating seal side face and the elasticity of the material allows it to be shaped around a wireline without severing it. The steel plates, being rigid, require the channels to be cut in them or it may either prevent a good seal forming or may damage/sever the wireline.
The elastomeric material immediately surrounding the wireline may be subject to the direct pressure and potentially elevated temperature from the fluid in the wellbore i.e. the steel reinforcing plates may not wholly cover that section of material. The elastomeric material may then be prone to flow or quasi-liquefy about the wireline and thereby eventually breaking the seal after sufficient exposure time.
The wireline valve rams and associated inner seals are designed such that when sealing under well pressure conditions the rams are energised by well pressure rather than hydraulic actuators. The rams have an amount of running clearance between the actuator and ram body which enables the rams to move independent of the actuator, typically about ⅛″ linear movement. Furthermore, the inner seals have a portion at the back where rubber protrudes beyond the supporting plates in a backwards direction, ensuring that the pressure face between ram body and inner seal is made up wholly of rubber. This ensures that the ram is able to maintain rubber pressure even in the event of rubber flow/loss over the cable interface as the ram will continually move to compensate for lost rubber volume. Thus inner seals can accommodate a certain amount of rubber loss without loss of seal integrity. Restricting this loss of rubber critical in the design of inner seals.